1. Field of the Invention
The present invention relates to a charger for charging an image carrier, an image forming apparatus of electrophotographic type incorporating the charger, and a charge control method for controlling charge of the image carrier.
2. Description of the Related Art
In a conventional image forming apparatus including an electrophotographic machine, such as an electrostatic copier, a printer, and facsimile, an image is formed as follows. First, after the surface of a photoconductor is uniformly charged by a contact charger, an electrostatic latent image is formed on the surface of the photoconductor by an exposure unit. Then, the electrostatic latent image is developed by a developing unit into a toner image and transferred directly or through an intermediate transfer member onto a transfer material such as a paper by a transfer unit. Finally, the toner image on the transfer medium is fixed by a fixing unit.
There is known, as a conventional image forming apparatus, one that brings a charging brush (primary charging member) into contact with a photoconductor to perform primary charge and then brings a charging roller which is provided on the downstream side relative to the charging brush in the rotational direction of the photoconductor into contact with the photoconductor to perform secondary charge so as to uniformly and satisfactorily charge the surface of the photoconductor (see, e.g., JP-A-2005-215321). The charger of this image forming apparatus uses the charging roller to eliminate irregular charge occurring when the charging member provided on the upstream side in the rotational direction of the photoconductor is used to charge the surface of the photoconductor, thereby achieving uniform charge of the photoconductor.
However, in the charger disclosed in JP-A-2005-215321, when the charging brush is used for a long time, impurities such as toner are adhered to or between brush bristles. This deteriorates charging efficiency of the charging brush. In order to cope with this problem, a voltage higher than a voltage required to charge an image carrier for image formation is applied to the charging brush. However, in this case, a large amount of current flows between the image carrier and charging brush, inducing an electric stress at the photosensitive layer of the image carrier. This reduces the film thickness of the photosensitive layer of the image carrier. In this case, contacting state of the brush bristles of the charging brush with the image carrier varies between the brush bristles. Accordingly, the magnitude of the reduction in the film thickness of the photosensitive layer of the image carrier varies in the axial direction of the image carrier. Further, in charging of the image carrier by double-contact charge using the charging brush and charging roller which are both brought into contact with the image carrier, the film thickness of the photosensitive layer of the image carrier is reduced also in accordance with the increase in the number of copies of image outputs.
The above reduction in the film thickness of the photosensitive member of the image carrier and variation in the magnitude of the reduction prevents the irregular charge resulting from primary charge from being satisfactorily eliminated by secondary charge using the charging roller. Therefore, the absolute value of the surface potential of the photosensitive layer may be reduced to a value lower than the absolute value of a preset image writing potential in some position in the axial direction on the image carrier. When the absolute value of the surface potential of the photosensitive layer 2a is reduced to a value lower than the absolute value of a preset image writing potential as described above, the relevant part is unsatisfactorily charged and appears on a print image as vertical lines to deteriorate image quality.